The invention relates generally to optical parametric oscillators (OPOs) and more particularly to a continuously tunable OPOs operating in the near-infrared and mid-infrared range, in both continuous wave (CW) and pulsed modes.
Optical parametric oscillators have been recognized as useful to effect the efficient conversion of fixed wavelength laser radiation into broadly wavelength-tunable radiation. OPOs can provide an efficient source of high power coherent radiation at wavelengths, which are not covered by conventional lasers. They convert monochromatic laser radiation (pump) into a tunable output via three-wave mixing process with quantum conversion efficiencies of up to >90%. The heart of an OPO is a nonlinear-optical (NLO) crystal which is characterized by a NLO coefficient, deff. In the NLO crystal, the pump photon decays into two less energetic photons (signal and idler) so that the sum of their energies is equal to that of the pump photon. In terms of optical frequencies and wavelengths, this is expressed as:ωp=ωs+ωi,  (1a)1/λp=1/λs+1λi,  (1b)where ωp, ωs, and ωi are the pump, signal and idler frequencies, which are related to the corresponding wavelengths λm as ωm=2π/λm. (here m stands for p, s, or i).
An important further constraint is that the sum of the signal and idler wave-vectors (k-vectors) must equal to that of the pump-momentum conservation or ‘phase-matching’ condition [A. Yariv: Quantum Electronics, 3rd ed. (Wiley, New York 1988)].kp=ks+ki,  (2)where kp, ks, and ki are the pump, signal and idler k-vectors, which are related to the corresponding wavelengths λm as km=2πnm/λm, where nm is the refractive index for each wave (m stands for p, s, or i).
The latter condition is never satisfied in the transparency range of isotropic media, where normal dispersion applies, but can be fulfilled in birefringent crystals through angle tuning. Alternatively, it can be fulfilled in “quasi-phase-matched” (QPM) crystals with periodically modulated nonlinearity (typical example is periodically-poled lithium niobate), where the artificially created grating of optical nonlinearity compensates for the wave-vector mismatch [M. M. Fejer, G. A. Magel, D. H. Jundt, R. L. Byer: Quasi-phase-matched 2nd harmonic-generation, Tuning and tolerances, IEEE J. Quantum Electron. 28, 2631-2654 (1992)].
Rotating the crystal (in the case of birefringent phase matching) or changing the quasi-phase-matched (QPM) orientation-reversal period (in the case of QPM crystals) changes the ratio between the signal and idler photon energies through phase-matching condition. This tunes the frequency of the output [I. T. Sorokina and K. L. Vodopyanov (Eds.), Solid-State Mid-Infrared Laser Sources (Springer, Berlin, 2003)]. Alternatively, the same goal can be achieved (in both birefringent and QPM crystals) by changing the crystal's temperature.
Wide OPO acceptance bandwidths can be achieved near OPO degeneracy point (that is when the signal and the idler beams have the same optical frequency) at the condition of type-I or type-0 phase matching (when the signal and the idler beams have the same polarization) [A. J. Campillo et al., U.S. Pat. No. 4,349,907 (September 1982); R. C. Slater, U.S. Pat. No. 7,023,545 B2 (April 2006)]. This means that an optical parametric oscillator (OPO), an optical parametric generator (OPG, a traveling-wave parametric device), or an optical parametric amplifier (OPA), can generate (or amplify) a broad range of frequencies simultaneously.
Moreover, one can maximize the gain bandwidth of the optical parametric device at degeneracy by carefully choosing the pump wavelength, crystalline orientation and/or QPM period. Specifically, the largest gain bandwidth for parametric process can be achieved near a certain wavelength λ0, such that the group-velocity dispersion of a NLO crystal near this wavelength is close to zero:d2k/dω2≈0,  (3)and if the wavelength of the pump laser source is chosen to be equal to half of that wavelength, λp=λ0/2. [A. Birmontas, A. Piskarskas, and A. Stabinis, Sov. J. Quantum Electron. 13, 1243 (1983)].
In this case, an anomalously broad, approximately octave-wide gain bandwidth around the degenerate signal-idler wavelength λ0, can be obtained for both angle-phase-matched crystals, e.g. ZnGeP2 (ZGP) [K. L. Vodopyanov, V. G. Voevodin, Opt. Commun. 117, 277 (1995)] and quasi-phase-matched crystals, e.g. GaAs [P. S. Kuo, K. L. Vodopyanov, M. M. Fejer, D. M. Simanovskii, X. Yu, J. S. Harris, D. Bliss, D. Weyburne, Opt. Lett. 31, 71 (2006)].
The condition (3) for the broadband OPA operation is formally equivalent to the condition:d2λp/dλs2≈0,  (4)described in [G. Imeshev et al., U.S. Patent Application US2005/0238070 A1 (October 2005)].
In the prior art work, a narrow-linewidth OPG tunable over the 14.8 to 18.5 μm region was described by Campillo et al. [A. J. Campillo et al., U.S. Pat. No. 4,349,907 (September 1982)]. The drawbacks of the setup are that it is very complex: it consists of 3 different NLO crystals and uses a 2-stage frequency conversion process to achieve mid-IR output, hence it needs an energetic laser pumping source (energy 10 mJ and few ps pulsewidth), in addition, it has very low conversion efficiency, on the order of 10−4, which limits it practical application. Also, the invention does not mention the idea of working close to the zero group-velocity dispersion (3).
A broadband source based on parametric device was described by Slater [R. C. Slater, U.S. Pat. No. 7,023,545 B2 (April 2006)], which can be used for chemical identification by flash spectroscopy. The invention uses the idea of a wide OPO acceptance bandwidth near degeneracy, but does not mention the condition (3) for achieving the highest bandwidth. Accordingly, the projected broadband output has a bandwidth of only ˜200 cm−1, which is much less than an octave.
There is a need for a system that allows broadband tuning of the output radiation with an option for fast tuning.